The proposed research aims to identify both the molecular mechanisms that underlie and the evolutionary processes that have produced functional divergence within gene expression networks between two closely related species. This will be initiated by experiments designed to measure gene expression across the genome for genotypes of Drosophila simulans that have had a segment of their genome replaced with the homologous segment from the closely related species Drosophila mauritiana. Identification of genes with patterns of expression in these genotype that are different from either parent species will provide insight into aspects of heterospecific interactions on gene expression. This will also provide candidate for the positional cloning of factors responsible for novel gene expression in the heterospecific genotypes. Once these factors have been identified, functional and comparative methods will be used to determine the molecular mechanisms underlying the novel expression. Finally, intraspecific genotypic and phenotypic variation within both species at loci implicated in these interactions will be obtained to identify the evolutionary forces that have led to the fixation of the incompatible alleles in these two species. [unreadable] [unreadable]